1. Field of the Invention
This invention relates generally to wireless signal transmission systems and, more specifically, to an Improved Signal Receiver Having Wide Band Amplification Capability.
2. Description of Related Art
In a conventional infrared transceiver system 10 depicted by the diagram of FIG. 1, infrared signals 14 are received by an infrared diode 12. These incident infrared signals 14 generate a current within the infrared diode 12, which is conventionally converted to a voltage signal by shunting the system with resistor RS as shown. This relatively low-voltage signal is then passed through a voltage amplifier 16. The signal then passes through various stages of staged amplification 18 before being carried on out of the system as the output signal VIRRX. What should be appreciated is at node VOUT the signal is essentially the incident IR signal 14, plus any noise created by the IR diode 12 or the resistor RS. It should be apparent that the better the signal-to-noise ratio at VOUT, the better and cleaner the amplification through the voltage amplifier 16 and the subsequent staged amplification 18.
Now turning to FIG. 2, we can discuss the operation of the conventional system in more depth. FIG. 2 is a schematic of a single-ended version of a conventional infrared transceiver system of FIG. 1. As can be seen in FIG. 2, the IR diode 12 is simulated by current source I1 and capacitance C1. RS of FIG. 1 is here R7, shunted with the current source. Essentially, what we have in this diagram is a current mirror 20 and a voltage amplifier 22. What should be appreciated from this circuit is that in normal operation the typical input level for fast infrared (FIR) frequency bandwidth will result in approximately 0.5 micro amps of current at current source I1, which results in 106 micro volts across a “real” 212 ohm resistor R7. Under such conditions, the resistor R7 will have a thermal noise of 17.8 micro volts (at 40 MHz frequency bandwidth), which results in a noise ratio of 15.5 decibels without even having entered the amplification stages. If we now look at the operation of the amplifier 22, we can see that typically, it is a high impedance voltage amplifier. The problem with this type of voltage amplifier is that R7, which is required for the specified system bandwidth, also provides additional noise that is added to the incident infrared signal 14 (at VOUT) before the signal is amplified—this further decreases the signal-to-noise ratio. It should also be understood that since the “Miller Effect” will apply to the input stage, the value of the intrinsic gate-to-drain capacitance of such a stage is multiplied by the voltage gain. For example, a voltage gain of 10 will result in a “Miller Effect” drain-to-gate capacity of 11 times. In order to achieve the desired bandwidth, a Cascode stage becomes a necessity. The addition of this Cascode stage results in a corresponding addition of another transistor-based noise contribution discussed above (i.e. a total of two equal noise-contributing stages). Consequently, this phenomena further degrades the signal to noise ratio and harms the amplifier performance. Another type of amplifier has been conventionally used, in which R7 is replaced by a feedback resistor. This amplifier has not been discussed herein, since its design is limited to a lower bandwidth, in particular, because of its poor noise performance.
Now turning to FIG. 3, we can see a preferred model for the prior art circuit of FIG. 2. FIG. 3 is a simulation of the circuit of FIG. 2 provided for the purposes of modeling the performance of the circuit; the pertinent results of this modeling are shown in FIGS. 4 and 5. FIG. 4 is a plot of noise vs. frequency bandwidth for the conventional circuit of FIGS. 1 through 3. As can be seen, at a frequency of approximately 40 MHz (which is in the FIR bandwidth), the spot noise is approximately 1.6×10−21√{square root over (Hz )}. This number will become more significant once we discuss the improvements of the present invention.
Now turning to FIG. 5 we can see the effect of these noises and capacitance's created in the prior art voltage feedback type amplification circuit. FIG. 5 is a response plot of output voltage (VIRRX) for the prior system of FIG. 2. As can be seen, the peaks and valleys are extremely erratic and choppy, which creates an unstable signal and ultimately inferior data processing. What is needed is an improved amplifier system to reliably handle in excess of 40 MHz frequency bandwidth.